The present invention generally relates to microwave energy interactive materials (“microwave interactive materials”) and, more specifically, to the shape of susceptors.
Microwave ovens are frequently used to heat food. As a result, the number of food items and constructs (e.g., packages) available for use with a microwave oven is increasing. It is well known for such a construct to include a layer of microwave energy interactive material (“microwave interactive material”) that is for interacting with microwave energy in a manner that reduces, enhances or otherwise alters the effectiveness of a microwave oven. There are several types of microwave interactive materials that have been used, including, but not limited to, susceptors and shields. Susceptors predominantly absorb microwave energy and thereby become hot, so that susceptors can be used to heat, brown and/or crisp at least a portion of an adjacent food item, such as through radiant heat transfer. In contrast, shields predominantly reflect microwave energy, so that shields can be used to direct microwave energy away from a portion of an associated food item to thereby restrict heating.
Shields are typically made of aluminum foil that is relatively thick as compared to the layer of microwave interactive material of a susceptor; therefore, shields typically have greater electrical conductivity than susceptors. As a result of the relatively high electrical conductivity, electrical arcing can occur at any pointed corners of a shield that is being exposed to microwave energy. In order to avoid this problem, it is common for shields to include rounded corners. In contrast, because of the relatively low electrical conductivity of susceptors, arcing typically does not occur at pointed corners of susceptors used in microwave ovens. Therefore, susceptors typically have pointed corners.
A typical susceptor includes a layer of microwave interactive material (e.g., a metal) secured to or supported on a support layer (e.g., paper or a polymeric film). A construct (e.g., a tray with an upright peripheral rim, flat tray, sleeve, wrap, carton or bag, such as for popping popcorn) that is for supporting food being cooked in a microwave oven often includes a bottom that includes a susceptor.
In an effort to promote uniform cooking, some microwave ovens include a turntable that has a rotatable disk-shaped tray (i.e., a tray that is at least generally round). A construct is typically upon the rotating disk-shaped tray during cooking, so that the lower surface of the construct's bottom is in opposing face-to-face contact with the upper surface of the turntable tray. The upper surface of the construct's bottom typically can include a susceptor, so that the susceptor is adjacent the food carried by (e.g., contained by) the construct. The susceptor absorbs microwave energy and becomes hot, such as for heating, browning, and/or crisping the food adjacent thereto. Depending upon the insulating characteristics of the construct's bottom, some of the heat provided by the hot susceptor may be transferred to the turntable tray. In particular, corners of the susceptor may be proximate the periphery of the turntable tray, and heat transferred from those corners of the susceptor to the turntable tray can lead to damaging thermally induced stress in the turntable tray, namely proximate the periphery of the turntable tray. This stress can result, for example, in the breaking of the turntable tray, as will be discussed in greater detail below.
FIG. 1 is a schematic top plan view of a bottom panel 20 of a prior art package that is upon a turntable tray 22. All of the turntable trays 22 referred to in this specification are conventional, made of glass (e.g., Pyrex brand glass), ceramic or the like, and are for use in a microwave oven (not shown). The lower surface of the bottom panel 20 is in opposing face-to-face contact with the upper surface of the turntable tray 22. The entirety of the upper surface of the bottom panel 20 is covered with a continuous layer of microwave interactive material. The continuous layer of microwave interactive material is schematically represented by stippling in FIG. 1. The microwave interactive material is operative for becoming hot when exposed to microwave energy. The enhanced heating rate of the microwave interactive material causes the center of the turntable tray 22 (i.e., those portions of the turntable tray that are covered by the bottom panel 20 and, therefore, the microwave interactive material) to heat faster than the outer areas of the turntable tray (i.e., those portions of the turntable tray that are not covered by the bottom panel 20). Accordingly, the turntable tray 22 can be characterized as having a hot center that is being constrained by a relatively cooler perimeter, so that the center of the turntable tray 22 is in compression and the perimeter of the turntable tray 22 is in tension.
The bottom panel 20 illustrated in FIG. 1 is also schematically illustrative of a susceptor (e.g., a susceptor patch) in isolation, with the support layer of the susceptor (support layers of susceptors are discussed in greater detail below with reference to FIG. 3C) corresponding in shape and size to the bottom panel, and the support layer being entirely covered with a continuous layer of microwave interactive material. The dashed lines in FIG. 1 designate a quadrant of the susceptor/bottom panel 20.
FIGS. 2A-G are schematic illustrations that respectively show theoretical thermally induced stress in turntable trays 22a-g of different sizes. Each of these figures schematically shows the location of a quadrant of the susceptor 20 (FIG. 1) upon a quadrant of the respective turntable tray 22a-g, and the susceptor is centered on the trays. The relatively dark, perpendicular lines in FIGS. 2A-G represent the periphery of the quadrant of the susceptor 20. The contrasting crosshatching in FIGS. 2A-G is illustrative of theoretical thermally induced stress in the turntable trays 22a-g. Each of these figures includes a legend for providing an understanding of how the contrasting crosshatching is illustrative of the stress. The illustrated thermally induced stress is the result of the susceptor 20 being upon the turntable trays 22a-g, and the microwave interactive material of the susceptor 20 absorbing microwave energy and thereby becoming hot and heating the turntable trays 22a-g. 
As apparent from FIGS. 2A-G, with the susceptor 20 centered, the relatively large diameter turntable trays have a peak thermally induced stress that is located inwardly from the perimeters of the turntable trays; and in contrast, the relatively small diameter turntable trays have a peak thermally induced stress that is located at, or proximate, the perimeters of the turntable trays.
It is been suggested that some consumers have a habit of placing relatively small packages with susceptors off-center on relatively large turntable trays in microwave ovens, because they believe that this arrangement enhances cooking. Even a relatively small susceptor that is sufficiently off-center on a relatively large turntable tray can cause the peak thermally induced stress to be disadvantageously located at, or proximate, the perimeter of the turntable tray.
It can be disadvantageous to have peak thermally induced stress that is located at, or proximate, the perimeters of turntable trays. For example, fractures and cracks tend to initiate at the edges of turntable trays because stress-enhancing defects, such as chips, are common at the edges of turntable trays. Bringing stress-enhancing defects and the peak thermally induced stresses together increases the possibility of fracturing turntable trays made of glass, and the like.
Accordingly, it is desirable for susceptors to function in a manner that seeks to keep peek thermally induced stress away from the perimeters of the turntable trays. At the same time, it is desirable to optimize the heating effectiveness of susceptors. Therefore, it is desirable to provide susceptors and other constructs that provide a new balance of properties.